It is well known in the art to use light transmitted through or reflected from a medium in order to determine characteristics of the medium. For example, in the medical field, where non-invasive physiological monitoring of vital signs of a patient is often required, light transmitted through a portion of the body, and reflected or scattered from the body surface may be measured to determine information about the patient. This type of non-invasive measurement is comfortable for the patient and can be performed rather quickly.
For example, during surgery, blood pressure, heart rate, breathing rate and blood oxygen saturation are often monitored. Moreover, for some individuals, there may be a daily, even hourly need to measure such parameters to know the individuals health and/or to detect and treat some diseases.
Furthermore, information about vital signs can also be important to individuals involved in athletic training and physical exercising. For example, one of the important applications related to physical activity is continuous heart rate monitoring. This field still requires developments in a sense that a suppressive majority of nowadays optical sensors performing heart rate monitoring must be attached to body parts, which is inconvenient as well as relatively unreliable, mainly due to the dependency on motion-artifacts. Other kinds of related applications are related to blood pressure monitoring, oximetry, breathing rate monitoring, etc. All of these applications are an essential part of controlled physical activity, for example, in the process of heart rehabilitation. Accordingly, the most common requirement for all of the corresponding monitoring devices is the ability to be stable, compact, sensitive and reliable under operation with batteries.
A number of optical monitoring techniques have been proposed in the art that use light as an optical signal transmitted through a medium, such as a portion of a blood perfused body tissue with the goal of determining vital signs. Generally, such a monitoring system (also known as a photoplethysmograph) includes a transmitter utilizing a probe clipped on a part of the body (e.g., a finger, forehead, ear pinna or an earlobe) that includes an optical source, e.g., a light emitting diode (LED) or a laser, for irradiating the body part with light placed on one side of the of the body part while a photodetector is placed on an opposite side of the body part. Typically, the conventional systems operate with the optical signal that is either a continuous wave light or a train of optic pulses all having a constant repetition frequency and a constant width.
The monitoring system also includes a receiver utilizing an optical photodetector (e.g., a photo diode) positioned in an optical path so that it has a field of view which ensures the capture of a portion of the light which is transmitted, reflected or scattered from the body part. The optical detector converts the light (i.e., optical signal) into an analog electrical signal, which is subsequently amplified and provided to an analyzer to retrieve information that was present in the optical signal. The information present in the optical signal can be both the information inserted by the transmitter as well as the information about the medium.
An example of the medical monitoring device using light transmitted through a portion of the blood perfused body tissue is a pulse oximeter. Pulse oximetry is used to determine the oxygen saturation of arterial blood. Oxyhemoglobin mainly absorbs infrared light while deoxyhemoglobin mainly absorbs visible red light. Accordingly, pulse oximeter devices typically contain two types of light sources, either light emitting diodes or laser diodes, operating in the red band of light and in the infrared band of light, respectively. Pulse oximeter devices also include photo-detectors for each of above mentioned wavebands and the processing unit that detects the ratio of red/infrared absorption and calculates the patient's oxygen saturation of arterial blood.
Devices employing optical sensors to detect heart pulse rate are also known. For example, U.S. Pat. No. 5,807,267 describes a device that can be worn on the wrist. The device includes a LED producing a constant light output which is focused on the radial artery of a person. Pulsations of the radial artery (caused by the pumping action of the heart) cause the walls of the radial artery to expand and contract at the heart rhythm rate. These pulsations cause variations (modulation) in the amount of light being reflected from the surface of the artery to the photo sensitive surface of a photo transistor. The photo transistor converts the changes in the received light level to a varying electrical signal which is amplified and filtered. The filtered signal is then supplied to the analog-to-digital (A/D) converter and transformed into a digital word. The digital word is then processed into numerical results which are displayed on a liquid crystal display (LCD) as heart beats per minute.
The data provided to the analyzer that include not only information about vital signals but also various noise data. In particular, the sources of noise may be separated on two main groups, such as electrical noise, and optical noise. The electrical noise is naturally caused by the electrical circuits, whereas the optical noise includes natural light source noises, ambient light along with motion artifacts due to non-perfect optical coupling between the human body and the light source and the photo-detector.
Specifically, transmission of optical energy as it passes through the body is strongly dependent on the thickness of the material through which the light passes, or the optical path length. Many portions of a patient's body are typically soft and compressible. Therefore, when the patient moves, the thickness of material through which optical energy passes can change. This results in the changes of the optical path length. For example, if optical energy passes through a finger and the user of an optical device moves in a manner which distorts or compresses the finger, the optical path length changes. Changes in the optical path length together with the changes of venous blood movement through during motion can produce enough distortion in the measured signal to make it difficult or impossible to determine desired information.
Since a patient generally moves in an erratic fashion, the compression of the finger is erratic. This causes the change in optical path length to be erratic, making the absorption erratic, resulting in a difficult to interpret measured signal. Furthermore, in order to generate a strong output signal intensity, probes utilizing light transmitted though a medium are designed to maximize contact between the light source (e.g., LED) and the medium as well as the photodetector and the medium to promote strong optical coupling between the LED, the medium, and the photodetector. In this way, a strong, clear signal can be transmitted through the medium only when the body is generally motionless.
The artifacts produced by body motion can be concurrent with the heart pulse and detected by the sensor as noise. In some cases, this noise can produce signals of amplitude sufficient to completely mask the heart pulse signal which is to be measured. Accordingly, many prior art optical probes are designed for use only when a user (e.g., patient) is relatively motionless since, as discussed above, motion induced noise can grossly corrupt the measured signal.
Several attempts are known to design devices, which would operate with the light scattered from a portion of the user's body. Since thickness of the material reflecting the light affects the optical path length in the less extends then in the case when the light passes though the medium, these devices are less prone to motion artifacts. Nevertheless, in the case of the scattered light, the processing of the signal received by the photodetector is even more sophisticated task, due to a rather low signal-to-noise ratio resulting in a low stability of the monitoring. The physical explanation of this phenomenon is rather straightforward. Since the useful signal in the reflection/scattered geometry is based entirely on coming-back photons, these photons have to experience a lot of scattering acts and have a rather high probability to be absorbed on their way, and mainly forward scattering dominates. Accordingly, the angle of a single scattering is small, and a lot of scattering acts are required in order to increase the signal to noise ratio and maintain the receiver of the monitoring system in the dynamic range.
In this connection, the applicant is not familiar with any technique that provides robust data of vital signs by using scattered light measurements. For example, U.S. Pat. Appl. Pub. No. 2009/0227853 describes an ear hook plethysmography (PPG) sensor and/or pulse oximetry (SpO2) sensor that can be attached to the skin in the regions of superficial artery and vein and posterior auricular artery and vein around the ear. Accordingly, an ear wearable heart rate monitor can be constructed with these sensors. Although US2009/0227853 alleges that these sensor systems should be less vulnerable to motion artifacts under motion conditions such as running and exercising, this application does not describe, inter alia, how to eliminate the noise associated with the variations of optical coupling and the influence of an ambient light on the useful information signal.